The Three Sons

Riddle: A wise man was on his deathbed. He called his three sons into his room and posed a riddle to them, explaining that whosoever answers his riddle to his satisfaction would claim his inheritance. The three sons listened intently as the father spoke, "What is as round as a circle and as straight as a line? It costs you no money, but if you lose it, all is lost. It can only be seen from certain angles. You can heat it up, but this is only what fools would do. What is it?" The eldest brother, trim as possible and taller than certain trees, spoke first. "Why father, the thing of which you speak is a human life, which encompasses infinite experiences but only stretches out in one direction. It can only be seen in retrospect and only fools would live in too hot a climate." However, the father frowned. "You were always too trim for riddles," he scolded. The second brother, wide as two men, spoke next, loudly and confidently: "Of course, our dear father is describing time. Time is infinite, and all things to all people, so it fits these descriptions. The mention of fools 'heating it up' is a reference to Hot Time, a disgusting local custom." The father at him sadly. "You have done a bad thing, answering in this way." The youngest brother, medium in every way, save for his very expensive hat, spoke last. "Father's words describe an abstract concept, but I know better, owing to my time as a man of society and influence. Father describes a weird window. It is circular, but has some straight lines on it. It costs no money, because it was already installed in the house you moved into, but all is lost because you'll lose your deposit if it is stolen. It can only be seen from certain angles, due to an error with the window, and only a fool would heat a window up. That is my answer." The father smiled. "That is also not correct. None of you have answered my riddle." The father then died peacefully, and his sons fell into poverty, where they also died peacefully. Who was correct?